Improved methods for miniaturization of integrated circuits have permitted the integration of millions of transistor circuit elements into a single silicon embodied circuit. Such a circuit is typically referred to as an integrated circuit chip or semiconductor die.
Semiconductor dies are created from a silicon wafer through the employment of various etching, doping and depositing steps that are well known in the art. Ultimately, the semiconductor die may be packaged by forming an encapsulant around the semiconductor die so as to form an "integrated circuit package" having a variety of pin-out or mounting and interconnection schemes. Plastic is often advantageously utilized as an encapsulant.
Integrated circuit packages that utilize plastic as an encapsulant are less expensive than other packaging options and provide performance and reliability that is acceptable for a number of different applications. Although plastic integrated circuit packages are more susceptible to external influences than ceramic or metallic packaging, they account for approximately 80% of the integrated circuit packages manufactured worldwide.
Integrated circuit packages come in a variety of configurations. For example, Dual-In-Line ("DIP") packaging provides an integrated circuit package having dual parallel rows of leads extending from the bottom for connection and mounting to an underlying printed circuit board. As a further example, Quad Flat Pack ("QFP") packaging provides an integrated circuit package having leads that extend from all four sides of the package. It follows that QFP integrated circuit packages permit a higher number of leads when compared to DIP integrated circuit packages. More compact packages of integrated circuits, which allow greater density on a printed circuit board, include Single-In-Line ("SIP") packaging, Pin Grid Array ("PGA") packaging, and Small Outline ("SO") packaging.
A semiconductor die is comprised of many interconnected transistors and associated passive circuit elements that perform one or more functions. These functions may be random access memory ("RAM"), central processing ("CPU"), communications, etc. Different types of semiconductor dies are employed to create a machine such as a personal computer. Combining semiconductor dies requires electrically connecting semiconductor dies with one another, as well as to devices such as keyboards, video monitors and printers. In order to accomplish these connections, conductive paths must be made available to connect a semiconductor die to external electrical circuits.
An array of electrical conductors called a "leadframe assembly" forms the conductive paths between a semiconductor die and external circuitry for facilitating interconnection therebetween. The leadframe assembly consists of a set of leadframe fingers. On one end, each leadframe finger is connected to the semiconductor die. For instance, in the case of a lead-on-chip package, each leadframe finger is designed to align with and connect to one of a series of connection pads that are located on the face of a semiconductor die. These connection pads are the points at which all input and output signals, as well as power and ground connections, are made for the semiconductor die to function as designed.
The lead tips of the leadframe assembly, being external to the integrated circuit package and extending from the leadframe fingers, are further connected to external circuitry such as a printed wiring board or another substrate. Alternatively, the lead tips of an integrated circuit package may be connected to an intermediate package such as a hybrid circuit or a multiple chip module. Hybrid circuits are typically ceramic substrates with thick- or thin-film metallization, while multichip modules utilize either ceramic or silicon substrates with built-up multilayer dielectric insulators and metal conductors to achieve the required system performance.
A heatsink is ordinarily provided beneath a semiconductor die. A heatsink functions to dissipate the heat generated by the semiconductor die. To aid in the dissipation of heat, integrated circuit packages can be fabricated to expose one or more surfaces of the heatsink from the integrated circuit package. An exposed surface of the heatsink is referred to as the "external surface" of the heatsink. In other words, it is advantageous for a surface of a heatsink to not be covered by encapsulant.
In order to avoid encapsulant from forming over the external surface of a heatsink, the external surface must be kept in close communication with the mold cavity in which the heatsink is placed. Current encapsulating processes rely on liquid plastic viscosity and the gravity of the heatsink to preclude any movement of the heatsink while inside the mold cavity. Gravity forces the external surface of the heatsink to make contact with the inner surface of the mold cavity. In doing so, gravity attempts to act as a sealing force. However, a poor sealing consistency has resulted from reliance on the weight of a heatsink. This is so since the hydraulic forces induced by the encapsulant can offset the gravity force. The hydraulic forces tend to displace the heatsink and leadframe assembly inside the mold cavity. As a consequence, encapsulant often infiltrates between the external surface of the heatsink and the inner surface of the mold cavity. Such encapsulant is referred to as "plastic flash" and "bleed". As a result, additional assembly steps are now necessary to remove the unwanted encapsulant from the external surface of the heatsink.
Other disadvantages are encountered by relying upon the force of gravity and viscosity of the encapsulant to expose the surface of a heatsink. First, in order for gravity to be applied correctly, the external surface of a heatsink must face downward when encapsulation takes place. This has the effect of significantly eliminating flexibility in the assembly process. Second, damage to a semiconductor die and its fragile internal connections can occur when the semiconductor die is displaced within a mold cavity.
In sum, typical integrated circuit packages that the applicant is aware of are unable to adequately preclude encapsulant from forming on the intended external surface of a heatsink. Rather, the viscosity of the encapsulant and the weight of the semiconductor die and heatsink are relied upon. Such reliance can, and often times does, result in the accumulation of unwanted encapsulant on the external surface of the heatsink. Thus, the prior art that the applicant is aware of fails to provide a satisfactory manner of fabricating an integrated circuit package having a heatsink with an external surface, without undertaking additional steps to remove encapsulant from the portion of the heatsink that is desired to be exposed.